However, my wife’s account (different email address with Yahoo, worked perfectly under V9) now don’t get downloaded. Instead it appears to be a mirror of my emails, including folder structure and so on. Her emails are arriving in Yahoo Mail but it appears that eM Client is failing to look at that account and just uses the primary account (mine) when refreshing emails.
Sounds like your wife’s account is using your Yahoo email account OAuth token login when the authentication part happened in V10 causing the mirror emails of yours, where normally when you upgrade to V10 and open each account you get asked which account you want to authenticate.
So if your Wife’s Yahoo account setup is an IMAP (non POP) setup, and she had nothing in eM Client Local folders, suggest to remove your wife’s Yahoo account/s and then readd her Yahoo account via the automatic account wizard via “Menu / Accounts”, where you should at the end of the account wizard setup then get asked to allow her Yahoo email address.
If that still doesn’t ask you to allow / select her Yahoo email account near the end of the account wizard, then go into her Yahoo account and “remove the eM Client OAuth token” and then close and reopen eM Client and that should then fix it.